ON PHYSICAL OPTICS. 445 



- The most singular of all the phenomena of refraction is perhaps tlie property 

 of some natural substances, which have a double eftect on the light transmitted 

 tlirough them, as if two mediums of different densities freely pervaded each 

 other, the one only acting on some of the rays of light, the other on the remain- 

 ing portion. These substances are usually crystallized stones, and their refrac- 

 tions have sometimes no further peculiarity; but the rhomboidal crystals of 

 calcarious spar, commonly called Iceland crystals, possess the remarkable 

 property of separating such pencils of light, as fall perpendicularly on them, 

 into two parts, one of them only being transmitted in the usual manner, 

 the other being deflected towards the greater angle of the crystal. It appears 

 from the experiments of Huygens, confirmed and extended by Dr. Wollas- 

 ton, that the medium, which causes the unusual refraction, has a different 

 refraqtive power, according to tlae direction in which the light passes through 

 it, and that if an oblate or flattened spheroid be described within a crystal, 

 its axis being in the middle of one of the obtuse solid angles, and its princi- 

 pal diameters in the proportion of 9 to 10, the refractive power, with respect 

 to light passing in any direction, will always be inversely as the diameter 

 of the spheroid which is parallel to it; and where it is greatest, will be equal 

 to that of the medium wliich produces the usual refraction, of which the 

 index is ^. A ray of light, falling perpendicularly on any surface of the spar, 

 its point of incidence being considered as the centre of the spheroid, will 

 meet the surface of the spheroid at the point where it is parallel to that of 

 the spar; and a ray incident on the same surface in any other direction, will 

 preserve a relation to the perpendicular ray, which is nearly the same as in 

 ordinary refraction. (Plate XXIX. Fig. 435.) 



It is also remarkable, that tbe two portions of light, thus separated, will not 

 be further subdivided by a transmission through a second piece, provided 

 that this piece be in a position parallel to that of the first; but if it be placed 

 in a transverse direction, each of the two pencils will be divided into 

 two others; a circumstance Avhieh appears to be the most unintelligible 

 of any that has been discovered respecting the phenomena of double re- 

 fraction. 



The appearances of colours, which are protluced by transparent plates of 



